1. Field of the Invention
This invention relates to surgical needles that are fabricated from alloys that display shape memory and to methods for using such surgical needles.
2. Description of the Related Art
Shape memory, as the name implies, refers to the characteristic of certain metals that if the metals are deformed, they can be returned to their original shape simply by heating. Metal alloys that display this "shape memory effect" have been known for decades. Although early applications for these materials were primarily military, more recently the alloys have been used for a variety of applications, including medical.
Although a number of alloys display the shape memory effect, NiTi alloys (called Nitinol alloys) have found the greatest utility. Alloys that are approximately 50 (atomic) percent Ni and 50 percent titanium have found particularly widespread use.
U.S. Pat. No. 4,170,990, issued Oct. 16, 1979 to Baumgart et al., discloses a method for implanting into living tissue a mechanical connecting element fabricated from a shape memory alloy and ultimately removing the elements. Before implantation, the element is given a shape memory at a high temperature, then cooled and deformed for insertion. After implantation, the element is heated above a particular temperature to cause it to assume the shape it had prior to the plastic deformation. When the element is to be removed, it is cooled and returned to the shape it had for insertion.
U.S. Pat. No. 4,425,908, issued Jan. 17, 1984 to Simon, discloses a Nitinol blood clot filter, which in its low temperature phase can be straightened and passed through a tube or catheter, while being kept cool. When it reaches its destination in the body (e.g., the vena cava), the element is transformed to its high temperature state and takes on a remembered filter shape.
U.S. Pat. No. 4,485,816, issued Dec. 4, 1984 to Krumme discloses a shape memory surgical staple. The staple is first given a desired closed shape at an elevated temperature. It is then cooled and deformed into an open position. The open staple is then positioned so that it extends between and is in contact with portions of tissue to be joined together. On heating the staple, it assumes its remembered (closed) shape, penetrating and joining the tissue.
U.S. Pat. No. 4,926,860, issued May 22, 1990, to Stice et al., discloses arthroscopic instrumentation that takes advantage of the "superelasticity" inherent in shape memory alloys. Superelasticity (or "pseudoelasticity") describes the property of shape memory alloys that they return to their original shape upon unloading after a substantial deformation. The instrumentation may include a cannula having a curved lumen and a flexible shape memory alloy probe. The curved cannula is inserted into the body with its distal end adjacent an operative site. The probe is given a straight shape in the austenitic condition or the cold worked martensitic condition. Forced through the cannula, the probe bends elastically, but when it emerges from the distal end, it resumes its straight shape. The instrumentation permits a straight probe to reach a remote operative location that might otherwise be inaccessible to the probe. The probe may comprise a pair of SME needles attached at their proximal ends to opposite ends of a suture. Similar technology is disclosed in U.S. Pat. No. 4,984,581, issued Jan. 15, 1991 to Stice.
U.S. Pat. No. 5,002,563, issued Mar. 26, 1991 to Pyka et al., discloses sutures utilizing shape memory alloys. The sutures are given a loop shape at an elevated temperature, then cooled and deformed (straightened) to facilitate insertion into tissue. After the suture is in place, it is heated or otherwise transformed to its high temperature (loop) shape. Although the suture is generally attached to a needle for insertion, it can alternatively be sharpened to obviate the need for a needle. That technique presupposes that the alloy can be sharpened and penetrate tissue in its low temperature state.